(A) Field of the Invention
The present invention relates to an over-current protection device, and more particularly, to an over-current protection device with high hold current.
(B) Description of the Related Art
Because the resistance of conductive composite materials having a positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance will instantaneously increases to a high resistance state (e.g. at least above 104 ohms), which is the so-called trip. Therefore, the over-current will be eliminated so as to protect the cell or the circuit device.
Over-current protection devices applied in high temperature environments must meet the following two criteria: (1) the time-to-trip cannot be too fast, for example, it must be larger than 2 seconds under the circumstance of 30 A and 80° C.; and (2) it must be capable of being tripped at room temperature 25° C. and 12 A. Generally, the over-current protection devices are of small sizes, and the PTC device using carbon black as the conductive fillers cannot easily meet the above requirements. Experiments using nickel powder as the conductive filler have also been conducted; however, hydrofluoric acid is generated when the nickel powder is mixed with fluorine-containing polymer at high temperature, and thus nickel powder cannot be used.
For example, over-current protection devices applied to motor vehicles must have superior heat dissipation capability as the vehicle is often insolated under strong sunshine. Conventionally, the resistance of the device using carbon black as the conductive filler is high, thus the hold current Ihold thereof is small (hold current is the largest current without trip). Consequently, the heat dissipation efficiency cannot be increased effectively.
In view of the above, it is urgent to make a breakthrough in ways to improve the heat dissipation efficiency of the over-current protection device to meet the above two characteristics, in order to meet the requirements for high temperature environment, such as automotive applications.